Résumé
Populus euramericana cv 1214 is a clone of poplar widely planted in Europe. It is a fast growing species that produces white wood of low density appreciated in the veneer industry. However, it has defects : logs often split after felling, and veneers curve and twist after drying. The defects can be related to high levels of growth stresses and high heterogeneities of physical and mechanical properties, such as drying shrinkage and wood stiffness, due to tension wood. Our study aims at characterizing the properties of tension wood that seem to be the most important for explaining the afore-mentioned defects. Residual direction of stress at the stem periphery in standing trees was measured with a sensor based on the single hole method. Macroscopic distribution of tension wood has been studied in disks, stained with zinc iodo-chloride. The agreement between this macroscopic staining and characterization of tension wood on microtomed sections was checked. Logs (located under the disks) were peeled; drying shrinkages were measured in small veneers with an optical device, after a (measured) hygro-thermal recovery in order to release the greatest part of growth strains before drying. Tension wood seenis to he more frequently observed in leaning trees and in slender trees with small crowns. Tension wood is distributed in small patches grouped in generally one angular area. Longitudinal drying shrinkage is higher (about 0.5%) in tension wood than in normal (about 0.1%). Variations in tangential shrinkage seem not to be significant. The hygrothermal recovery has no significant influence on drying shrinkages. The residual maturation strains are higher in tension wood. The value of strains measured can be explained by the surface of G fibers located under the measurement, but the relation varies among trees. Both properties (maturation strains and drying shrinkages) are continuously distributed from 'sub-normal'wood with very small longitudinal maturation strain and drying shrinkage to very pronounced tension wood with high longitudinal maturation strain and drying shrinkage. Therefore, the usual classification into two categories 'normal'and 'tension' wood gives only poor information on wood properties and continuous quantitative data are preferable. (Résumé d'auteur)